Customize the core Calendar block time format

The default time format for the core Calendar block depends on the locale, but you can customize it using a custom block template. In the custom block template you can add a timeFormat property option with the time format you prefer.

Steps to customize the default calendar time format:

copy view.php
concrete\blocks\calendar\view.php

navigate to application\blocks and create a folder called "calendar"
application\blocks\calendar

inside the "calendar" folder, create a folder called "templates"
application\blocks\calendar\templates